Generally, since in a robot, and in particular, an articulated robot, there is an error between parameters describing a geometrical construction of robot arms such as robot parameters held in a control device, that is, an angle between links in the origin posture, a turning angle between links, a distance between links, a link length, etc., and actual parameters of the robot, whereby an error is produced between the position and posture of the robot, which are recognized by the control device, and the actual position and posture of the robot. The position and posture of the robot which are recognized by the control device are compared with the actual position and posture of the robot, wherein the robot parameters which are held by the control device are corrected, and the correction of the error is called "calibration" or "calibrating".
As a first example of the prior arts of the robot calibration, there is a method disclosed in Japanese Laid-open Patent Publication No. 274213 of 1994. This is described as follows:
A plurality of reference points are fixed around a robot, a plurality of different postures are taught so that points of a tool attached to the wrist portion of a robot for each of the reference points are made coincident with these reference points, and the teaching points are registered. Repeating this procedure, teaching points equivalent to the quantity (the number of reference points x the number of postures) are registered. The correction amount is calculated by using the data of these teaching points.
However, with the first prior art art, since it is necessary that an operator operates keys of a teaching box while watching the tip end of a tool and carries out a micro positioning of a robot, there is a shortcoming, by which the operation is very cumbersome and takes much time.
The second example of the prior arts in order to solve this shortcoming was disclosed by Japanese Laid-open Patent Publication No. 261682 of 1993. This is described as follows:
This method is such that a calibrating device which has a six-axis degree of freedom in action and has displacement detecting feature for the degree of freedom in each axis is mounted in a place where the relative positional relationship from a robot is already known or can be accurately measured, and the tip end portion of the calibrating device is mechanically connected to the tip end portion of the wrist of the robot, whereby robot calibration is carried out by inversely calculating position data and posture data of the robot based on position data and posture data obtained from the displacement detecting means of the calibrating device.
However, there were the following problems in the second example of prior arts.
(1) Since the calibrating device has displacement detecting features consisting of six degree of freedoms, it is complicated and expensive. PA0 (2) In addition, it is difficult to accurately measure the relative positional relation and posture between the calibrating device and a robot.